Optical coupling apparatus and methods of making same

ABSTRACT

Disclosed are apparatus and methods for optical coupling in optical communications. In one embodiment, an apparatus for optical coupling is disclosed. The apparatus includes: a planar layer; an array of scattering elements arranged in the planar layer at a plurality of intersections of a first set of concentric elliptical curves crossing with a second set of concentric elliptical curves rotated proximately 90 degrees to form a two-dimensional (2D) grating; a first taper structure formed in the planar layer connecting a first convex side of the 2D grating to a first waveguide; and a second taper structure formed in the planar layer connecting a second convex side of the 2D grating to a second waveguide. Each scattering element is a pillar into the planar layer. The pillar has a top surface whose shape is a concave polygon having at least 6 corners.

BACKGROUND

Optical gratings are frequently used to couple light between a waveguideand an optical fiber. Due to extremely different dimensions of thewaveguide and the optical fiber, direct coupling would incur tremendouslight loss. It is thus essential to meticulously design a waveguidelight coupling apparatus for light mode field matching to the fiberdimension.

For example, an incoming light to a waveguide is usually in an unknownand arbitrary polarization state, such that a polarization splittinggrating coupler (PSGC) is needed to provide polarization light in eithertransverse magnetic (TM) or transverse magnetic (TE) polarization modefrom the optical fiber to the waveguide. The coupling efficiency of aPSGC is typically impacted by a polarization dependent loss (PDL) of TEand TM modes, which may result from non-zero fiber angle used tominimize reflections at the interface between fiber and grating. Aconventional PSGC includes circular or square scattering elements at theintersection of grating lines on the grating, which results in highpolarization dependent loss between TE and TM modes and degradescoupling efficiency.

As such, there exists a need to develop a method and apparatus ofefficient optical coupling using optical gratings.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of illustration.

FIG. 1 illustrates an exemplary block diagram of a device, in accordancewith some embodiments of present disclosure.

FIG. 2A illustrates a top view of an exemplary two-dimensional (2D)grating coupler, in accordance with some embodiments of the presentdisclosure.

FIG. 2B illustrates an exemplary scattering element in a 2D gratingcoupler, in accordance with some embodiments of the present disclosure.

FIG. 3A illustrates a top view of an exemplary 2D grating coupler withan optical fiber, in accordance with some embodiments of the presentdisclosure.

FIG. 3B illustrates an exemplary incident angle of an optical fiber to a2D grating coupler, in accordance with some embodiments of the presentdisclosure.

FIG. 4A illustrates a top view of a portion of a 2D grating coupler, inaccordance with some embodiments of the present disclosure.

FIG. 4B illustrates a cross-sectional view of a portion of a 2D gratingcoupler, in accordance with some embodiments of the present disclosure.

FIGS. 5A-5K illustrate cross-sectional views of a portion of anexemplary grating coupler at various stages of a fabrication process, inaccordance with some embodiments of the present disclosure.

FIG. 6 illustrates a top view of an exemplary 2D grating coupler withapodization scattering patterns, in accordance with some embodiments ofthe present disclosure.

FIG. 7A illustrates a top view of another exemplary 2D grating coupler,in accordance with some embodiments of the present disclosure.

FIG. 7B illustrates some exemplary scattering elements in a 2D gratingcoupler, in accordance with some embodiments of the present disclosure.

FIG. 8 illustrates a top view of yet another exemplary 2D gratingcoupler, in accordance with some embodiments of the present disclosure.

FIG. 9 illustrates a top view of still another exemplary 2D gratingcoupler, in accordance with some embodiments of the present disclosure.

FIG. 10 illustrates additional exemplary scattering elements in a 2Dgrating coupler, in accordance with some embodiments of the presentdisclosure.

FIG. 11 illustrates exemplary coupling efficiency performances of adisclosed 2D grating coupler and a conventional 2D grating coupler, inaccordance with some embodiments of the present disclosure.

FIG. 12 illustrates a flow chart of an exemplary method for making anexemplary 2D grating coupler, in accordance with some embodiments of thepresent disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, it will be understood that when anelement is referred to as being “connected to” or “coupled to” anotherelement, it may be directly connected to or coupled to the otherelement, or one or more intervening elements may be present.

A waveguide surrounded by a cladding layer may confine light based onrefractive index contrast between the materials in the waveguide and thecladding layer. For example, a silicon waveguide with sub-microndimension can confine infrared light (with a wavelength larger thanabout 700 nanometers or 700 nm) due to its strong refractive indexcontrast to its silicon oxide cladding layer, wherein the refractiveindices for silicon and silicon oxide are about 3.47 and 1.45,respectively. In optical systems, e.g. a data communication system usinga light with a wavelength equal to about 1310 nm, or a telecommunicationsystem using a light with a wavelength equal to about 1550 nm, a siliconwaveguide usually has a height at about 200 to 350 nm, and has a widthat about 370 to 470 nm, to ensure good single-mode light transmission.To receive or transmit light signals, light needs to be coupled betweena waveguide and an optical fiber. But an optical fiber has a dimensionat least 30 times larger than that of a waveguide. For example, asingle-mode fiber typically has a diameter of at least 8 micrometers.Due to the extremely different dimensions of the waveguide and theoptical fiber, direct coupling would incur tremendous light loss. It isthus essential to meticulously design a waveguide light couplingapparatus for light mode field matching to the fiber dimension. While anoutgoing light from a silicon waveguide is usually in transversemagnetic (TE) mode and can be vertically coupled to a fiber using singlepolarization grating coupler, an incoming light to a silicon waveguideis usually in an unknown and arbitrary polarization state, such that apolarization splitting grating coupler (PSGC) is needed to providepolarization light in either transverse magnetic (TM) or transversemagnetic (TE) polarization mode from the optical fiber to the waveguide.

In one embodiment, a PSGC may be a two-dimensional (2D) grating couplerformed by two single polarization grating couplers nearly perpendicularto each other. Each single polarization grating coupler has ellipticalgrating lines with the major axis parallel to the fiber azimuth. Thetotal grating area of the PSGC is larger than a core size of the fiber.The PSGC includes scattering elements at the intersection of gratinglines on the 2D grating. Each scattering element may have a top surfacewhose shape is a concave polygon having at least 6 corners and/or atleast 8 edges. Different designs of the scattering elements aredisclosed to reduce the polarization dependent loss and improve lightcoupling efficiency from the optical fiber to the 2D grating coupler.

The sizes of the scattering elements on the disclosed grating couplermay be the same or different. In one embodiment, the scattering elementsbecome gradually larger along a first direction from a first convex sideof the 2D grating to a first concave side of the 2D grating, the firstconvex side being opposite to the first concave side; and the scatteringelements become gradually larger along a second direction from a secondconvex side of the 2D grating to a second concave side of the 2Dgrating, the second convex side being opposite to the second concaveside. This helps to improve the coupling efficiency of the gratingcoupler, because different scattering element sizes cause apodization tothe optical coupling, which increases mode field matching of the PSGC tothe fiber.

FIG. 1 illustrates an exemplary block diagram of a device 100, inaccordance with some embodiments of present disclosure. It is noted thatthe device 100 is merely an example, and is not intended to limit thepresent disclosure. Accordingly, it is understood that additionalfunctional blocks may be provided in or coupled to the device 100 ofFIG. 1, and that some other functional blocks may only be brieflydescribed herein.

Referring to FIG. 1, the device 100 comprises an electronic die 102, alight source die 104, a photonic die 106, an interposer 110 and aprinted circuit board (PCB) substrate 114. The electronic die 102, lightsource die 104 and the photonic die 106 are coupled together throughinput/output interfaces (not shown) on the interposer 110. In someembodiments, the interposer 110 is fabricated using silicon. In someembodiments, the interposer 110 comprises at least one of the following:interconnecting routing, through silicon via (TSV), and contact pads. Insome embodiments, the interposer 110 is to integrate all componentsincluding the electronic die 102, the light source die 104, and thephotonic die 106 together. In certain embodiments, each of the dies102/104/106 are coupled to the interposer 110 using a flip-chip (C4)interconnection method. In some embodiments, high density soldermicrobumps are used to couple the dies 102/104/106 to the interposer110. Further, the interposer 110 is coupled to the PCB substrate 114through wire bonding 112 or through silicon-vias (TSV) 116 usingsoldering balls. The TSVs 116 can comprise electrically conductive pathsthat extend vertically through the interposer 110 and provide electricalconnectivity between the electronic die 102 and the PCB 114. In someembodiments, the PCB substrate 114 can comprises a support structure forthe device 100, and can comprise both insulating and conductive materialfor isolation devices as well as providing electrical contact for activedevices on the photonic die 106 as well as circuits/devices on theelectronic die 102 via the interposer 110. Further, the PCB substrate114 can provide a thermally conductive path to carry away heat generatedby devices and circuits in the electronic die 102 and the light sourcedie 104.

In some embodiments, the electronic die 102 comprises circuits (notshown) including amplifiers, control circuit, digital processingcircuit, etc., as well as driver circuits for controlling the lightsource 104 or elements in the photonic die 106. In some embodiments, thelight source die 104 comprises a plurality of components (not shown),such as at least one light emitting elements (e.g., a laser or alight-emitting diode), transmission elements, modulation elements,signal processing elements, switching circuits, amplifier, input/outputcoupler, and light sensing/detection circuits. In some embodiments, thelight source die 104 is on the photonic die 106. In some embodiments,the photonic die 106 comprises an optical fiber array 108 attachedthereon, an optical interface and a plurality of fiber-to-chip gratingcouplers 118. In some embodiments, the plurality of fiber-to-chipgrating coupler 118 is configured to couple the light source 106 and theoptical fiber array 108. In some embodiments, the optical fiber array108 comprises a plurality of optical fibers and each of them can be asingle-mode or a multi-mode optical fiber. In some embodiments, theoptical fiber array 108 can be epoxied on the photonic die 106.

In some embodiments, each of the plurality of fiber-to-chip gradingcoupler 118 enables the coupling of optical signals between the opticalfiber array 108 and the light source die 102 or correspondingphotodetectors on the photonic die 106. Each of the plurality offiber-to-chip grating couplers 118 comprises a plurality of gratings anda waveguide with designs to improve coupling efficiency between theoptical fiber on the corresponding waveguide, which are discussed indetails below in various embodiments of the present disclosure.

During operation, optical signals received from a remote server attachedon one end of the optical fiber array 108 can be coupled through thefiber-to-chip grating couplers 118 attached to the other end of theoptical fiber array 108 to the corresponding photodetectors on thephotonic die 106. Alternatively, optical signals received from the lightsource die 104 can be coupled through the fiber-to-chip grating couplers118 to the optical fiber array 108 which can be further transmitted tothe remote server.

FIG. 2A illustrates a top view of an exemplary two-dimensional (2D)grating coupler 200, in accordance with some embodiments of the presentdisclosure. As shown in FIG. 2A, the 2D grating coupler is formed by twosingle polarization grating couplers nearly perpendicular to each other.Each single polarization grating coupler has a respective taper regionand a shared grating region 230. The first single polarization gratingcoupler includes a first taper structure 210 and the shared gratingregion 230; and the second single polarization grating coupler includesa second taper structure 220 and the shared grating region 230.

In one embodiment, the first taper structure 210, the second taperstructure 220 and the shared grating region 230 are all formed in aplanar layer, which may be a semiconductor layer, e.g. a silicon layeron a silicon-on-insulator (SOI) substrate. In one embodiment, the firsttaper structure 210 is formed in the planar layer connecting a firstconvex side 212 of the 2D grating 230 to a first waveguide 218 in theplanar layer; and the second taper structure 220 is formed in the planarlayer connecting a second convex side 222 of the 2D grating 230 to asecond waveguide 228 in the planar layer.

As shown in FIG. 2A, each single polarization grating coupler haselliptical grating lines that are concentric elliptical curves. Thefirst single polarization grating coupler includes a first set ofconcentric elliptical curves 216; and the second single polarizationgrating coupler includes a second set of concentric elliptical curves226 that are rotated proximately 90 degrees to form a two-dimensional(2D) grating. The two single polarization grating couplers share thegrating region 230 including the 2D grating and an array of scatteringelements 232 arranged in the planar layer at a plurality ofintersections of the first set of concentric elliptical curves crossingwith the second set of concentric elliptical curves. Any numbers ofelliptical curves in each single polarization grating coupler and anynumbers of scattering elements 232 on each elliptical curve can be usedand are within the scope of the present disclosure.

In a first embodiment, the grating coupler 200 scatters incident lightreceived from the first waveguide 218 in a direction perpendicular tothe grating curves 216 along the radius direction A1, a direction fromthe first convex side 212 of the 2D grating to a first concave side 214of the 2D grating, the first convex side 212 being opposite to the firstconcave side 214. In a second embodiment, the grating coupler 200scatters incident light received from the second waveguide 228 in adirection perpendicular to the grating curves 226 along the radiusdirection A2, a direction from the second convex side 222 of the 2Dgrating to a second concave side 224 of the 2D grating, the secondconvex side 222 being opposite to the second concave side 224. In eitherthe first embodiment or the second embodiment, the incident light isscattered out of the 2D grating, which includes periodic gratings formedby the array of scattering elements.

In a third embodiment, the 2D grating coupler 200 scatters incidentlight received from a fiber having a core size 250 attached to the 2Dgrating. The 2D grating coupler 200 is configured for splitting theincident light received from the fiber on top of the planar layer to aparallel polarization component and an orthogonal polarizationcomponent. In this embodiment, the first waveguide 218 comprises a firstoutput port located substantially at a focal point of the first set ofelliptical curves 216; and the second waveguide 228 comprises a secondoutput port located substantially at a focal point of the second set ofelliptical curves 226. The 2D grating coupler 200 couples the parallelpolarization component to the first output port in the first waveguide218 via the first taper structure 210; and couples the orthogonalpolarization component to the second output port in the second waveguide228 via the second taper structure 220. Alternatively, the 2D gratingcoupler 200 can couple the orthogonal polarization component to thefirst output port in the first waveguide 218 via the first taperstructure 210; and couples the parallel polarization component to thesecond output port in the second waveguide 228 via the second taperstructure 220.

As shown in FIG. 2A, the first taper structure 210 has a reducing firstwidth from the first convex side 212 to the first waveguide 218; and thesecond taper structure 220 has a reducing second width from the secondconvex side 222 to the second waveguide 228. In one embodiment, thefirst taper structure 210 is configured for transmitting a first portionof the incident light from the fiber to the first waveguide 218 toachieve a minimum insertion loss; and the second taper structure 220 isconfigured for transmitting a second portion of the incident light tothe second waveguide 228 to achieve a minimum insertion loss. The firstportion of the incident light is substantially a parallel polarizationcomponent of the incident light, and the second portion of the incidentlight is substantially an orthogonal polarization component of theincident light. Each of the parallel polarization component and theorthogonal polarization component comprises a polarized light split fromthe incident light. The polarized light has either a transverse magnetic(TM) polarization mode or a transverse magnetic (TE) polarization mode.

As shown in FIG. 2A, in the first set of concentric elliptical curvesand the second set of concentric elliptical curves of the 2D grating,each elliptical curve has an equal spacing relative to an adjacentconcentric elliptical curve, where the spacing may be configured as agrating period of the 2D grating.

In one embodiment, each scattering element is a pillar into the planarlayer, where the pillar has a top surface whose shape is a concavepolygon. FIG. 2B illustrates an exemplary scattering element 232 in a 2Dgrating coupler, e.g. the 2D grating coupler 200 in FIG. 2A, inaccordance with some embodiments of the present disclosure. Thescattering element 232 in FIG. 2B has a shape of a concave polygonhaving 2 reflex interior angles 235, 236 and 8 edges in total. Theconcave polygon has reflection symmetry about a line 234 crossing the 2reflex interior angles to divide the concave polygon into two equalconvex pentagons. As shown in FIG. 2B, the concave polygon has 6 cornersthat do not have reflex interior angles.

As shown in FIG. 2B, the concave polygon has 2-fold rotational symmetry;but has no N-fold rotational symmetry, when N is larger than 2. That is,the concave polygon will look exactly the same after a rotation by anangle of 360°/2=180°; but will look exactly the same after a rotation byan angle of 360°/N, when N is larger than 2.

FIG. 3A illustrates a top view of an exemplary 2D grating coupler 200with an optical fiber 310, in accordance with some embodiments of thepresent disclosure. As discussed above, both the optical fiber 310 andthe 2D grating coupler 200 may be attached to or included in a photonicdie on a substrate. The 2D grating coupler 200 includes an array ofscattering elements 232 on the photonic die for transmitting lightbetween the photonic die and the optical fiber 310.

In one embodiment, a total area of the array of scattering elements 232in the top surface of the planar layer is slightly larger than a coresize 250 of the optical fiber 310 and is determined based on a diameterof the optical fiber 310. In one example, when the optical fiber 310 hasa diameter of about 8 to 10 micrometers, the core size 250 of theoptical fiber 310 is about 15 to 20 micrometers.

In one embodiment, the 2D grating 230 of the coupler 200 is configuredfor receiving an incident light from the optical fiber 310 with anincident angle 320, as shown in FIG. 3B. The incident angle 320 ismeasured in plane of incidence between an axis of the optical fiber 310and the Z direction, a direction perpendicular to the planar layer. Theplane of incidence is a plane which contains the surface normal of theplanar layer and the propagation vector of the incident light. That is,the plane of incidence is the plane formed by the Z direction and the Xdirection.

Referring to both FIG. 2B and FIG. 3B, the line 234 is along the Ydirection, which is a direction in a top surface of the planar layer andperpendicular to the plane of incidence of an incident light from theoptical fiber 310. In one embodiment, each scattering element 232 has afirst length along the Y direction, and has a second length along the Xdirection that is in the top surface and perpendicular to the Ydirection. In one embodiment, a ratio of the second length to the firstlength may be determined based on the incident angle 320. As shown inFIG. 2B, the concave polygon is symmetric about a line 234 along the Ydirection and is symmetric about a line 233 along the X direction. Insome embodiments, the incident angle 320 can be configured in a range of5-15 degrees according to the structure, geometry, pattern, and materialproperties of the 2D grating coupler 200 including the scatteringelements 232.

Polarized light with its electric field along the plane of incidence isreferred to as transverse-magnetic (TM) polarized, while light whoseelectric field is normal to the plane of incidence is calledtransverse-electric (TE) polarized. FIG. 2B also shows the direction ofthe electric field of an incident light, when the incident light is TMand TE polarized respectively. As shown in FIG. 2B, the electric fieldof an incident light is along the X direction when the incident light isTM polarized; and is along the Y direction when the incident light is TEpolarized.

In one embodiment, the incident angle of the incident light from thefiber is zero. Each scattering element in this embodiment may have ashape of a concave polygon that is in the top surface of the planarlayer and has a 4-fold rotational symmetry. That is, the concave polygonwill look exactly the same after a rotation by an angle of 360°/4=90°.In this case, the ratio of the second length to the first length isequal to one.

In another embodiment, the incident angle is non-zero; and the ratio ofthe second length to the first length is larger than one. As such, eachscattering element in this embodiment may have a shape of a concavepolygon that is in the top surface of the planar layer and does not havea 4-fold rotational symmetry. That is, the concave polygon in thisembodiment will not look exactly the same after a rotation by an angleof 360°/4=90°. In other embodiments, the ratio of the second length tothe first length becomes larger as the incident angle becomes larger.

FIG. 4A illustrates a top view of a portion 400 of a 2D grating coupler,in accordance with some embodiments of the present disclosure. As shownin FIG. 4A, the grating portion 400 includes two scattering elements431, 432, adjacent to each other on a grating curve 416, which may beone of a set of concentric elliptical curves of a 2D grating region ofthe grating coupler.

In one embodiment, the scattering elements 431, 432 are formed in asemiconductor layer 430. In one embodiment, the scattering elements 431,432 comprise a dielectric material such as silicon oxide, while thesemiconductor layer 430 comprises a semiconductor material such assilicon.

FIG. 4B illustrates a cross-sectional view of the 2D grating couplerportion 400 along the direction A-A′ in FIG. 4A, in accordance with someembodiments of the present disclosure. In the illustrated embodiments,the 2D grating coupler 400 fabricated on a semiconductor substrate 410comprises a multi-layered structure comprising an insulation layer 420and a semiconductor layer 430.

In the illustrated embodiment, the semiconductor substrate 410 comprisessilicon. The insulation layer 420 comprises a dielectric material suchas silicon oxide, and is fabricated on the semiconductor substrate 410using chemical vapor deposition, physical vapor deposition, etc. In someembodiments, the insulation layer 420 can be replaced by other types ofdielectric materials, such as Si3N4, SiO2 (e.g., quartz, and glass),Al2O3, and H2O, according to various embodiments of the presentdisclosure.

In some embodiments, the semiconductor layer 430 comprises silicon andis deposited on the insulation layer 420 using chemical vapordeposition. In some embodiments, the semiconductor substrate 410, theinsulation layer 420 and the semiconductor layer 430 are formed as asilicon-on-insulator (SOI) substrate.

In some embodiments, the scattering elements 431, 432 are formedaccording to a predetermined pattern as shown in FIG. 2A. In someembodiments, the scattering elements 431, 432 are formed as part of acladding layer comprising silicon oxide. In some embodiments, thecladding layer can comprise other types of dielectric materialsaccording to different applications, including polycrystalline siliconand silicon nitride.

In some embodiments, the 2D grating coupler 400 may further comprise: abottom reflection layer that is located between the semiconductorsubstrate 410 and the insulation layer 420 and comprises at least one ofthe following: Al, Cu, Ni, and a combination; and/or a top reflectionlayer that is located on the cladding layer and comprises at least oneof the following: Al, Cu, Ni and a combination. In some embodiments, thetop reflection layer only covers the taper structures of the 2D gratingcoupler 400. In some embodiments, the taper structures (not shown) ofthe 2D grating coupler 400 comprise the same material used in thesemiconductor layer 430. In other embodiments, the taper structurescomprise a second material that is different from the material used inthe semiconductor layer 430.

FIGS. 5A-5K illustrate cross-sectional views of a portion of anexemplary grating coupler 500 at various stages of a fabricationprocess, in accordance with some embodiments of the present disclosure.FIG. 5A is a cross-sectional view of the grating coupler 500-1 includinga first layer 510 and a second layer 520 disposed on the first layer510, at one of the various stages of fabrication, according to someembodiments of the present disclosure. The first layer 510 may be formedof silicon or another semiconductor material as a substrate. The secondlayer 520 may be formed of silicon oxide or another oxide material as aninsulation layer.

FIG. 5B is a cross-sectional view of the grating coupler 500-2 includinga semiconductor layer 530 formed on the insulation layer 520 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The semiconductor layer 530 may be formed by anepitaxial growth of a semiconductor material, e.g. silicon, on theinsulation layer 520.

FIG. 5C is a cross-sectional view of the grating coupler 500-3 includinga hard mask layer 540 deposited on the semiconductor layer 530 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The hard mask layer 540 on the semiconductor layer530 may comprise an organic or inorganic material.

FIG. 5D is a cross-sectional view of the grating coupler 500-4 includinga photoresist layer 550 deposited on the hard mask layer 540 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The photoresist layer 550 on the hard mask layer 540may comprise a photoresist material.

FIG. 5E is a cross-sectional view of the grating coupler 500-5 includingpatterned portions of the photoresist layer 550, formed on the hard masklayer 540 at one of the various stages of fabrication, according to someembodiments of the present disclosure. The photoresist layer 550 ispatterned according a predetermined pattern, e.g. by removing portionscorresponding to the scattering elements shown in FIGS. 1-4, based onwaveguide lithography and development.

FIG. 5F is a cross-sectional view of the grating coupler 500-6 includingpatterned portions of the hard mask layer 540, formed at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. Because the photoresist layer 550 was patterned tohave openings over the hard mask layer 540, the portions of the hardmask layer 540 that are exposed by the photoresist layer 550 areremoved, e.g., via a wet or dry etch procedure.

FIG. 5G is a cross-sectional view of the grating coupler 500-7, wherethe photoresist layer 550 is removed at one of the various stages offabrication, according to some embodiments of the present disclosure.For example, the photoresist layer 550 may be removed by a resiststripping.

FIG. 5H is a cross-sectional view of the grating coupler 500-8 includingan array of etched regions 532, 534, formed at one of the various stagesof fabrication, according to some embodiments of the present disclosure.Because the hard mask layer 540 was patterned to have openings over thesemiconductor layer 530, the portions of the semiconductor layer 530that are exposed by the hard mask layer 540 are removed, e.g., via a wetor dry etch procedure, to form the array of etched regions 532, 534.

In some embodiments, surfaces of the etched regions 532, 534 may besmoothed by: oxidizing the silicon surfaces of the etched regions 532,534; etching the silicon oxide surfaces; and repeating the oxidizing andthe etching several times to smooth the surfaces of the etched regions532, 534.

FIG. 5I is a cross-sectional view of the grating coupler 500-9, wherethe hard mask layer 540 is removed at one of the various stages offabrication, according to some embodiments of the present disclosure.For example, the hard mask layer 540 may be removed by a resiststripping.

FIG. 5J is a cross-sectional view of the grating coupler 500-10including a cladding layer 560, which is formed at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. The cladding layer 560 may be formed by depositing adielectric material such as silicon oxide over the semiconductor layer530 and into the array of etched regions 532, 534.

FIG. 5K is a cross-sectional view of the grating coupler 500-11, wherethe top portion of the cladding layer 560 is polished at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. The top portion of the cladding layer 560 may bepolished to form an array of scattering elements 562, 564 in the arrayof etched regions 532, 534, e.g. based on a chemical-mechanicalpolishing process.

FIG. 6 illustrates a top view of an exemplary 2D grating coupler 600with apodization scattering patterns, in accordance with someembodiments of the present disclosure. The 2D grating coupler 600 inFIG. 6 is the same as the 2D grating coupler 200 in FIG. 2A, except thatthe 2D grating coupler 600 has a grating region 630 including 2D gratingformed by an array of scattering elements 632 that become graduallylarger along the A1 direction from a first convex side 612 of the 2Dgrating 630 to a first concave side 614 of the 2D grating 630, the firstconvex side 612 being opposite to the first concave side 614; and becomegradually larger along the A2 direction from a second convex side 622 ofthe 2D grating 630 to a second concave side 624 of the 2D grating 630,the second convex side 622 being opposite to the second concave side624.

In one embodiment, there is a same first distance between centers ofevery two adjacent scattering elements 632 along the A1 direction; andthere is a same second distance between centers of every two adjacentscattering elements 632 along the A2 direction. The first distance maybe equal to the second distance.

In another embodiment, the scattering elements 632 gradually change bothshapes and sizes along the A1 direction from the first convex side 612of the 2D grating 630 to the first concave side 614 of the 2D grating630; and gradually change both shapes and sizes along the A2 directionfrom the second convex side 622 of the 2D grating 630 to the secondconcave side 624 of the 2D grating 630.

In another embodiment, the scattering elements 632 become graduallylarger along the X direction from a first corner 635 of the 2D grating630 to a second corner 636 of the 2D grating 630. The second corner 636is opposite to the first corner 635. The second corner 636 is fartheraway from the first taper structure 218 and the second taper structure228 than the first corner 635.

FIG. 7A illustrates a top view of another exemplary 2D grating coupler700, in accordance with some embodiments of the present disclosure. The2D grating coupler 700 in FIG. 7 is the same as the 2D grating coupler200 in FIG. 2A, except that the 2D grating coupler 700 has a gratingregion 730 including 2D grating formed by an array of scatteringelements 732 each of which has a top surface whose shape is a concavepolygon having at least 6 reflex interior angles and at least 12 edges.

FIG. 7B illustrates some exemplary scattering elements in a 2D gratingcoupler, e.g. the 2D grating coupler 700 in FIG. 7A, in accordance withsome embodiments of the present disclosure. In one embodiment, eachscattering element 732 in FIG. 7A may have a shape of a concave polygon760 having 6 reflex interior angles and 12 edges in total. The concavepolygon 760 has reflection symmetry about a line along the X directionand about a line along the Y direction; and has 6-fold rotationalsymmetry. That is, the concave polygon 760 will look exactly the sameafter a rotation by an angle of 360°/6=60°. As shown in FIG. 7B, theconcave polygon 760 is divisible into a hexagon 769 and six triangles761, 762, 763, 764, 765, 766, located on six sides of the hexagon 769respectively. In one embodiment, the six triangles 761, 762, 763, 764,765, 766 are congruent. In one embodiment, the six triangles 761, 762,763, 764, 765, 766 are all regular triangles. In one embodiment, thehexagon 769 is a regular hexagon.

In another embodiment, each scattering element 732 in FIG. 7A may have ashape of a concave polygon 770 having 6 reflex interior angles and 12edges in total. The concave polygon 770 has reflection symmetry about aline along the X direction and about a line along the Y direction; has2-fold rotational symmetry; but has no N-fold rotational symmetry, whenN is larger than 2. As shown in FIG. 7B, the concave polygon 770 isdivisible into a hexagon 779 and six triangles 771, 772, 773, 774, 775,776, located on six sides of the hexagon 779 respectively. In oneembodiment, the six triangles 771, 772, 773, 774, 775, 776 include: fourregular triangles 771, 772, 775, 776 that are congruent and have a firstsize, and two regular triangles 773, 774 that are congruent and have asecond size larger than the first size.

In yet another embodiment, each scattering element 732 in FIG. 7A mayhave a shape of a concave polygon 780 having 6 reflex interior anglesand 12 edges in total. The concave polygon 780 has reflection symmetryabout a line along the X direction and about a line along the Ydirection; has 2-fold rotational symmetry; but has no N-fold rotationalsymmetry, when N is larger than 2. As shown in FIG. 7B, the concavepolygon 780 is divisible into a hexagon 789 and six triangles 781, 782,783, 784, 785, 786, located on six sides of the hexagon 789respectively. In one embodiment, the six triangles 781, 782, 783, 784,785, 786 include six isosceles triangles that are congruent. In oneembodiment, the six triangles 781, 782, 783, 784, 785, 786 include: fourtriangles 781, 782, 785, 786 that are congruent and not isoscelestriangles, and two isosceles triangles 783, 784 that are congruent.

In still another embodiment, each scattering element 732 in FIG. 7A mayhave a shape of a concave polygon 790 having 8 reflex interior anglesand 14 edges in total. The concave polygon 790 has reflection symmetryabout a line along the X direction and about a line 798 along the Ydirection; has 2-fold rotational symmetry; but has no N-fold rotationalsymmetry, when N is larger than 2. As shown in FIG. 7B, the concavepolygon 790 is divisible into an octagon 799 and six triangles 791, 792,793, 794, 795, 796. As shown in FIG. 7B, while the line 798 crosses twosides of the octagon 799, the six triangles 791, 792, 793, 794, 795, 796are located on the remaining six sides of the octagon 799 respectively.In one embodiment, the six triangles 791, 792, 793, 794, 795, 796include six isosceles triangles that are congruent. In one embodiment,the six triangles 791, 792, 793, 794, 795, 796 include: four triangles791, 792, 795, 796 that are congruent and not isosceles triangles, andtwo isosceles triangles 793, 794 that are congruent. One or more of thescattering elements in any one of FIGS. 1-7A can be replaced by ascattering element having a shape according to one of the abovedescribed polygons referring to FIG. 7B, according to variousembodiments of the present teaching.

FIG. 8 illustrates a top view of yet another exemplary 2D gratingcoupler 800, in accordance with some embodiments of the presentdisclosure. The 2D grating coupler 800 in FIG. 8 is the same as the 2Dgrating coupler 200 in FIG. 2A, except that the 2D grating coupler 800has a grating region 830 including 2D grating formed by an array ofscattering elements 832 each of which has a top surface whose shape is aconcave polygon having 4 reflex interior angles and 10 edges in total.

Referring to an expanded view of the scattering elements 832 in FIG. 8,each scattering element 832 may have a shape of a concave polygon havingat least one of the following properties. First, the concave polygon hasreflection symmetry about a line along the X direction and about a linealong the Y direction. Second, the concave polygon has 2-fold rotationalsymmetry; but has no N-fold rotational symmetry, when N is larger than2. Third, the concave polygon is divisible into a rectangle 860 and twotriangles 870, 880 located on two sides of the rectangle 860,respectively.

In one embodiment, the rectangle 860 is a square. In one embodiment, thetwo triangles 870, 880 are isosceles triangles that are congruent. Inanother embodiment, the two triangles 870, 880 are regular trianglesthat are congruent. One or more of the scattering elements in any one ofFIGS. 1-7A can be replaced by a scattering element having a shape asdescribed above referring to FIG. 8, according to various embodiments ofthe present teaching.

FIG. 9 illustrates a top view of still another exemplary 2D gratingcoupler 900, in accordance with some embodiments of the presentdisclosure. The 2D grating coupler 900 in FIG. 9 is the same as the 2Dgrating coupler 200 in FIG. 2A, except that the 2D grating coupler 900has a grating region 930 including 2D grating formed by an array ofscattering elements 932 each of which has a top surface whose shape is aconcave polygon having 4 reflex interior angles and 12 edges in total.

Referring to an expanded view of the scattering elements 932 in FIG. 9,each scattering element 932 may have a shape of a concave polygon havingat least one of the following properties. First, the concave polygon hasreflection symmetry about a line along the X direction and about a linealong the Y direction. Second, the concave polygon has 2-fold rotationalsymmetry; but has no N-fold rotational symmetry, when N is larger than2. Third, the concave polygon is divisible into a rectangle 960 and twohorizontal triangles 970, 980 located on two sides of the rectangle 960,respectively. Each horizontal triangle has two longer horizontal edgesand two shorter vertical edges.

In one embodiment, the rectangle 960 is a vertical rectangle. A verticaltriangle has two longer vertical edges and two shorter horizontal edges.In one embodiment, the rectangle 960 is a square. In one embodiment, thetwo horizontal triangles 970, 980 are congruent. In another embodiment,the two horizontal triangles 970, 980 are not congruent. One or more ofthe scattering elements in any one of FIGS. 1-8 can be replaced by ascattering element having a shape as described above referring to FIG.9, according to various embodiments of the present teaching.

FIG. 10 illustrates additional exemplary scattering elements in a 2Dgrating coupler, e.g. any one of the 2D grating couplers shown in FIGS.1-9, in accordance with some embodiments of the present disclosure.According to various embodiments, each scattering element of the 2Dgrating coupler may have a shape of a concave polygon 1010 having 8reflex interior angles and 16 edges in total. According to variousembodiments, the concave polygon 1010 has at least one of the followingproperties. First, the concave polygon 1010 has reflection symmetryabout a line along the X direction and about a line along the Ydirection. Second, the concave polygon 1010 has 8-fold rotationalsymmetry; but has no N-fold rotational symmetry, when N is larger than8. Third, the concave polygon 1010 is divisible into an octagon 1019 andeight triangles 1011, 1012, 1013, 1014, 1015, 1016, 1017, 1018, locatedon eight sides of the octagon 1019, respectively. In one embodiment, theoctagon 1019 is a regular octagon. In one embodiment, the eighttriangles 1011, 1012, 1013, 1014, 1015, 1016, 1017, 1018 include eightisosceles triangles that are congruent. In one embodiment, the eighttriangles 1011, 1012, 1013, 1014, 1015, 1016, 1017, 1018 include: fourtriangles 1011, 1012, 1015, 1016 that are congruent and not isoscelestriangles; two isosceles triangles 1013, 1014 that are congruent andhave a first size; and two isosceles triangles 1017, 1018 that arecongruent and have a second size smaller than the first size.

According to various embodiments, each scattering element of the 2Dgrating coupler may have a shape of a concave polygon 1020 having 8reflex interior angles and 16 edges in total. According to variousembodiments, the concave polygon 1020 has at least one of the followingproperties. First, the concave polygon 1020 has reflection symmetryabout a line along the X direction and about a line along the Ydirection. Second, the concave polygon 1020 has 8-fold rotationalsymmetry; but has no N-fold rotational symmetry, when N is larger than8. Third, the concave polygon 1020 is divisible into an octagon 1029 andeight triangles 1021, 1022, 1023, 1024, 1025, 1026, 1027, 1028, locatedon eight sides of the octagon 1029, respectively. In one embodiment, theoctagon 1029 is a regular octagon. In one embodiment, the eighttriangles 1021, 1022, 1023, 1024, 1025, 1026, 1027, 1028 include eightright triangles that are congruent. In one embodiment, the eighttriangles 1021, 1022, 1023, 1024, 1025, 1026, 1027, 1028 include: fourright triangles 1021, 1022, 1025, 1026 that are congruent; two isoscelestriangles 1023, 1024 that are congruent and have a third size; and twoisosceles triangles 1027, 1028 that are congruent and have a fourth sizesmaller than the third size. One or more of the scattering elements inany one of FIGS. 1-9 can be replaced by a scattering element having ashape as described above referring to FIG. 10, according to variousembodiments of the present teaching.

FIG. 11 illustrates exemplary coupling efficiency performances of adisclosed 2D grating coupler and a conventional 2D grating coupler, inaccordance with some embodiments of the present disclosure. In FIG. 11,the curve 1110 shows a coupling efficiency performance of a conventionalPSGC vs. a wavelength spectrum of operational wavelengths of theconventional PSGC with respect to a TE polarized incident light; thecurve 1120 shows a coupling efficiency performance of a conventionalPSGC vs. a wavelength spectrum of operational wavelengths of theconventional PSGC with respect to a TM polarized incident light; thecurve 1130 shows a coupling efficiency performance of a disclosed PSGCvs. a wavelength spectrum of operational wavelengths of the disclosedPSGC with respect to a TE polarized incident light; and the curve 1140shows a coupling efficiency performance of a disclosed PSGC vs. awavelength spectrum of operational wavelengths of the disclosed PSGCwith respect to a TM polarized incident light. The disclosed PSGC may beformed as any one of the 2D grating couplers disclosed in FIGS. 1-10. Asshown in FIG. 11, compared to the conventional PSGC, the disclosed PSGChas very close coupling efficiency performances between the TE and TMpolarization lights across the operational wavelength spectrum. At atarget center operational wavelength 1150, the disclosed PSGC has a muchsmaller polarization dependent loss than the conventional PSGC, becauseof the closer coupling efficiency performances between the TE and TMpolarization lights of the disclosed PSGC. In addition, the disclosedPSGC improves coupling efficiency performance compared to theconventional PSGC, because a minimum coupling efficiency of the TE andTM polarization lights of the disclosed PSGC is higher than that of theconventional PSGC, at the target center operational wavelength 1150.

FIG. 12 illustrates a flow chart of an exemplary method 1200 for makingan exemplary 2D grating coupler, e.g. any one of the 2D grating couplersdisclosed in FIGS. 1-10, in accordance with some embodiments of thepresent disclosure. At operation 1202, an insulation layer is formed ona semiconductor substrate. At operation 1204, a semiconductor materialis epitaxially grown on the insulation layer to form a semiconductorlayer. At operation 1206, a hard mask is deposited on the semiconductorlayer. At operation 1208, a photoresist is deposited on the hard mask.At operation 1210, a pattern is determined based on an array of concavepolygons. In various embodiments, the concave polygons may have a sameshape or different shapes, and/or may have a same size or differentsizes, according to the shapes and sizes disclosed in FIGS. 1-10. Thepattern may be chosen from the disclosed patterns based on desiredshape, geometry and materials of the gratings, as well as a desiredoperational wavelength range.

At operation 1212, the photoresist is patterned according to thepattern. At operation 1214, the hard mask is etched according to thepattern. At operation 1216, the semiconductor layer is etched to form anarray of etched regions. At operation 1218, the surfaces of the etchedregions are smoothed, e.g. by repetitively oxidizing the surfaces andetching the oxidized surfaces. At operation 1220, the etched hard maskon the semiconductor layer is removed. At operation 1222, a dielectricmaterial is deposited into the array of etched regions and over thesemiconductor layer. At operation 1224, the top dielectric material ispolished to form an array of scattering elements. The order of theoperations in FIG. 12 may be changed according to various embodiments ofthe present teaching.

In one embodiment, an apparatus for optical coupling is disclosed. Theapparatus includes: a planar layer; an array of scattering elementsarranged in the planar layer at a plurality of intersections of a firstset of concentric elliptical curves crossing with a second set ofconcentric elliptical curves rotated proximately 90 degrees to form atwo-dimensional (2D) grating; a first taper structure formed in theplanar layer connecting a first convex side of the 2D grating to a firstwaveguide; and a second taper structure formed in the planar layerconnecting a second convex side of the 2D grating to a second waveguide.Each scattering element is a pillar into the planar layer. The pillarhas a top surface whose shape is a concave polygon having at least 6corners.

In another embodiment, a system for communication is disclosed. Thesystem includes: a photonic die on a substrate; an optical fiberattached to the photonic die; and an array of scattering elements on thephotonic die for transmitting light between the photonic die and theoptical fiber. The array of scattering elements is arranged in a planarlayer at a plurality of intersections of a first set of concentricelliptical curves crossing with a second set of concentric ellipticalcurves rotated proximately 90 degrees to form a two-dimensional (2D)grating. The 2D grating is configured for receiving an incident lightfrom the optical fiber with an incident angle. The incident angle ismeasured in plane of incidence between an axis of the optical fiber anda direction perpendicular to the planar layer. Each scattering elementhas a first length along a first direction that is in a top surface ofthe planar layer and perpendicular to the plane of incidence, and has asecond length along a second direction that is in the top surface andperpendicular to the first direction. A ratio of the second length tothe first length is determined based on the incident angle.

In yet another embodiment, a method for forming an optical coupler isdisclosed. The method includes: forming an insulation layer on asemiconductor substrate; epitaxially growing a semiconductor material onthe insulation layer to form a semiconductor layer; etching thesemiconductor layer to form an array of etched regions in thesemiconductor layer according to a predetermined pattern; and depositinga dielectric material into the array of etched regions to form an arrayof scattering elements in the semiconductor layer. The scatteringelements are arranged at a plurality of intersections of a first set ofconcentric elliptical curves crossing with a second set of concentricelliptical curves rotated proximately 90 degrees to form atwo-dimensional (2D) grating. Each scattering element is a pillar in acorresponding one of the etched regions. The pillar has a top surfacewhose shape is a concave polygon having at least 8 edges.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. An apparatus for optical coupling, comprising: aplanar layer; an array of scattering elements arranged in the planarlayer at a plurality of intersections of a first set of concentricelliptical curves crossing with a second set of concentric ellipticalcurves rotated proximately 90 degrees to form a two-dimensional (2D)grating, wherein each scattering element is a pillar into the planarlayer, and the pillar has a top surface whose shape is a concave polygonhaving at least 6 corners; a first taper structure formed in the planarlayer connecting a first convex side of the 2D grating to a firstwaveguide; and a second taper structure formed in the planar layerconnecting a second convex side of the 2D grating to a second waveguide.2. The apparatus of claim 1, wherein the concave polygon is at least oneof: a polygon having 2 reflex interior angles and 8 edges; a polygonhaving 4 reflex interior angles and 10 edges; a polygon having 4 reflexinterior angles and 12 edges; a polygon having 6 reflex interior anglesand 12 edges; or a polygon having 8 reflex interior angles and 16 edges.3. The apparatus of claim 1, wherein the concave polygon is at least oneof: a polygon that has reflection symmetry about a line which dividesthe polygon into two convex pentagons; a polygon that is divisible intoa rectangle and two triangles located on two sides of the rectanglerespectively; a polygon that is divisible into a vertical rectangle andtwo horizontal triangles located on two sides of the vertical rectanglerespectively; a polygon that is divisible into a hexagon and sixtriangles located on six sides of the hexagon respectively, wherein thesix triangles include at least one of: six regular triangles that arecongruent, or four regular triangles that are congruent and have a firstsize, and two regular triangles that are congruent and have a secondsize larger than the first size; or a polygon that is divisible into anoctagon and six isosceles triangles, wherein: the octagon has reflectionsymmetry about a line crossing two sides of the octagon, and the sixisosceles triangles are congruent and located on the remaining six sidesof the octagon respectively.
 4. The apparatus of claim 1, wherein: thescattering elements become gradually larger along a first direction fromthe first convex side of the 2D grating to a first concave side of the2D grating, the first convex side being opposite to the first concaveside; and the scattering elements become gradually larger along a seconddirection from the second convex side of the 2D grating to a secondconcave side of the 2D grating, the second convex side being opposite tothe second concave side.
 5. The apparatus of claim 4, wherein: there isa same first distance between centers of every two adjacent scatteringelements along the first direction; and there is a same second distancebetween centers of every two adjacent scattering elements along thesecond direction.
 6. The apparatus of claim 5, wherein the firstdistance is equal to the second distance.
 7. The apparatus of claim 4,wherein: the scattering elements gradually change shapes and sizes fromthe first convex side of the 2D grating to the first concave side of the2D grating; and the scattering elements gradually change shapes andsizes from the second convex side of the 2D grating to the secondconcave side of the 2D grating.
 8. The apparatus of claim 1, wherein:the concave polygon has 2-fold rotational symmetry; and the concavepolygon has no N-fold rotational symmetry, when N is larger than
 2. 9.The apparatus of claim 1, wherein: the first taper structure has areducing first width from the first convex side to the first waveguide;the second taper structure has a reducing second width from the secondconvex side to the second waveguide; the first waveguide comprises afirst output port located substantially at a focal point of the firstset of elliptical curves; and the second waveguide comprises a secondoutput port located substantially at a focal point of the second set ofelliptical curves.
 10. A system for communication, comprising: aphotonic die on a substrate; an optical fiber attached to the photonicdie; and an array of scattering elements on the photonic die fortransmitting light between the photonic die and the optical fiber,wherein: the array of scattering elements is arranged in a planar layerat a plurality of intersections of a first set of concentric ellipticalcurves crossing with a second set of concentric elliptical curvesrotated proximately 90 degrees to form a two-dimensional (2D) grating,the 2D grating is configured for receiving an incident light from theoptical fiber with an incident angle, the incident angle is measured inplane of incidence between an axis of the optical fiber and a directionperpendicular to the planar layer, each scattering element has a firstlength along a first direction that is in a top surface of the planarlayer and perpendicular to the plane of incidence, and has a secondlength along a second direction that is in the top surface andperpendicular to the first direction, and a ratio of the second lengthto the first length is determined based on the incident angle.
 11. Thesystem of claim 10, wherein: the incident angle is zero; each scatteringelement has a shape of a concave polygon in the top surface of theplanar layer; and the concave polygon has a 4-fold rotational symmetry.12. The system of claim 10, wherein: the incident angle is non-zero; andthe ratio of the second length to the first length is larger than one.13. The system of claim 12, wherein the ratio of the second length tothe first length becomes larger as the incident angle becomes larger.14. The system of claim 10, wherein: each scattering element has a shapeof a concave polygon in the top surface of the planar layer; and theconcave polygon is symmetric about a line along the first direction andis symmetric about a line along the second direction.
 15. The system ofclaim 10, wherein: a total area of the array of scattering elements inthe top surface is slightly larger than a core size of the optical fiberand is determined based on a diameter of the optical fiber.
 16. Thesystem of claim 10, wherein: the scattering elements become graduallylarger along the second direction from a first corner of the 2D gratingto a second corner of the 2D grating; the second corner is opposite tothe first corner; and the second corner is farther away from the firsttaper structure and the second taper structure than the first corner.17. The system of claim 10, further comprising: a first taper structurethat is formed in the planar layer connecting a first convex side of the2D grating to a first waveguide and is configured for transmitting afirst portion of the incident light to the first waveguide to achieve aminimum insertion loss; and a second taper structure that is formed inthe planar layer connecting a second convex side of the 2D grating to asecond waveguide and is configured for transmitting a second portion ofthe incident light to the second waveguide to achieve a minimuminsertion loss, wherein the first portion of the incident light issubstantially a parallel polarization component of the incident light,the second portion of the incident light is substantially an orthogonalpolarization component of the incident light, each of the parallelpolarization component and the orthogonal polarization componentcomprises a polarized light split from the incident light, and thepolarized light has a transverse magnetic (TM) polarization mode or atransverse magnetic (TE) polarization mode.
 18. A method for forming anoptical coupler, comprising: forming an insulation layer on asemiconductor substrate; epitaxially growing a semiconductor material onthe insulation layer to form a semiconductor layer; etching thesemiconductor layer to form an array of etched regions in thesemiconductor layer according to a predetermined pattern; and depositinga dielectric material into the array of etched regions to form an arrayof scattering elements in the semiconductor layer, wherein thescattering elements are arranged at a plurality of intersections of afirst set of concentric elliptical curves crossing with a second set ofconcentric elliptical curves rotated proximately 90 degrees to form atwo-dimensional (2D) grating, wherein each scattering element is apillar in a corresponding one of the etched regions, and the pillar hasa top surface whose shape is a concave polygon having at least 8 edges.19. The method of claim 18, wherein: the scattering elements becomegradually larger along a first direction from a first convex side of the2D grating to a first concave side of the 2D grating according to thepredetermined pattern, the first convex side being opposite to the firstconcave side; the scattering elements become gradually larger along asecond direction from a second convex side of the 2D grating to a secondconcave side of the 2D grating according to the predetermined pattern,the second convex side being opposite to the second concave side; thereis a same first distance between centers of every two adjacentscattering elements along the first direction; and there is a samesecond distance between centers of every two adjacent scatteringelements along the second direction.
 20. The method of claim 19,wherein: the semiconductor material comprises silicon; the dielectricmaterial comprises silicon oxide; and the concave polygon is symmetricabout a line along the first direction and is symmetric about a linealong the second direction.